Fluid level sensor with stair step output

ABSTRACT

A two wire and a three wire resistive fluid level sensor for measuring fluid levels within a container are disclosed. The sensor includes a plurality of resistors connected in series. The sensor produces an output signal by providing a short circuit to the circuit nodes between the resistors so that a stair-step output signal is created as the float moves in conjunction with the liquid level in a container. The value of the resistances can be varied in order to accommodate various cross-sectional tank contours and thus produce a usable, readily and easily configurable sensor. The structure of the sensors enables measurement of depth variances far in excess of prior art sensors.

BACKGROUND OF THE INVENTION

Field of the Invention 1. This invention relates to sensors fordetecting fluid levels within a container, and more specifically tofluid level sensors for use in fuel tanks containing gasoline, diesel,or other volatile fuels.

The most commonly used fluid level sensor is the variable resistorsensor utilizing a float to produce a resistance change in the variableresistor. As the float moves vertically with the fluid level, theelectrical resistance of the sensor changes typically from 30 to 270ohms. In most sensors, a sliding or moving contact attached to the floatestablishes a resistive circuit based upon the position of the contactwith respect to a wirewound resistor or a thick film resistor printed onan insulating base or substrate.

Other approaches to fluid level detection include use of resistors withlarge temperature coefficients, known as thermistors, located at variousvertical positions in the fluid reservoir. As electrical power isapplied to the resistors, the devices immersed in the fluid remain coolwhile those that are exposed to air will increase in temperature andproduce a change in overall resistance of the device. Extensive signalconditioning and temperature compensation circuitry is typicallyrequired with such a sensor to create a usable signal. Fluidcompatibility and manufacturing costs limit widespread acceptance ofthis type of device.

A vertical sensor with a sliding contact has been used in someautomotive applications. Typically a float provides a contact point withrespect to a resistor. The resistor is usually a wire helix wound aboutan insulating mandrel.

Examples of prior art fluid level sensors are shown in the followingpatents: U.S. Pat. Nos. 4,637,254 to Dyben et al., 4,779,460 toCruickshank, 3,113,282 to Coleman, 3,106,693 to de Giers, 2,484,690 tode Giers, and Italian Patent No. 619,958 to Carlo Ceresa et al.

An example of thick film resistor technology used in a liquid levelsensor is shown in Weaver, U.S. Pat. No. 4,920,798. The Weaver deviceincludes a thick film resistive coated plate with a slidable contactmember providing a resistive signal in proportion to the position of thefloat.

Trucks and large vehicles which have fuel tanks with a depth of 50centimeters or more have experienced long-term reliability problems withfuel sensors. In addition, accuracy of fluid level detection is also aproblem. Further, vehicle design and space requirements may restrict themounting location for the sensor to the side or bottom of the fuel tankas a result of interferences. Leaks attributable to the mountinglocation and resistor failure due to vibration are common occurrences insuch applications. Many different physical sizes and operatinggeometries are required due to the significant number of different tankand vehicle mechanical designs. Typical tank depths for trucks may rangefrom 30 centimeters to 150 centimeters.

Off-road vehicles such as bulldozers, cranes, loaders and forkliftsoften are not equipped with fuel level sensors. Typically such vehiclesare produced in small quantities and the economics of producing sensorsfor these applications are cost prohibitive in view thereof. Theanticipated long service life and extreme vibration of physical movementof this type of equipment renders conventional sensors impractical. Tankdepths in this field vary from 60 centimeters to 200 centimeters. Sincemany of these tanks are long and narrow, the sensor may be subjected tosignificant wear as a result of constant fuel sloshing within the tank.Storage tanks used for petroleum products and other liquids often use amechanical or manually operated dipstick for measuring fluid levelwithin the tank. Electronic sensors and indicators have not generallybeen cost effective in this particular application. Tank materials maybe metal or plastic or combinations thereof, and tank depths may extendup to 4 meters.

A liquid level sensor for use in harsh environments suitable for largerliquid containers is needed to resolve the above difficulties whicharise in detecting fuel level within a container.

SUMMARY OF THE INVENTION

A fluid level sensor according to one embodiment of the presentinvention includes a conductive strip having a first and a secondsurface, an insulator attached to and substantially covering the firstsurface of the strip, first, second and third conductive segmentsmounted on the insulator in a substantially linear arrangement, a firstresistive device attached to the insulator and electrically connectedbetween the first conductive segment and the second conductive segment,a second resistive device attached to the insulator and electricallyconnected between said second conductive segment and said thirdconductive segment, a float including a hole therethrough for receivingthe strip, the float disposed about the strip and movable in response tothe fluid level in the container, and electrical contact means attachedto the float for electrically connecting the second surface of the stripand one of the first, second, or third conductive segments when thefloat is positioned about the strip.

One object of the present invention is to provide an improved fuel levelsensor for use in harsh environments.

Another object of the present invention is to provide a fuel levelsensor which is capable of detecting fluid level variances in excess ofthose of the prior art.

A further object of the present invention is to provide a fuel levelsensor which may be configured to provide a nonlinear output signal orresistance proportional with the cross-sectional contours of thecontainer in which the sensor is mounted.

Related objects and advantages of the present invention will become moreapparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of one embodiment of the fuellevel sensor according to the present invention.

FIG. 2 is a diagrammatic illustration of another embodiment of the fuellevel sensor according to the present invention.

FIG. 3 is a cutaway front elevational view of a fuel level sensoraccording to the present invention.

FIG. 4 is a top view of the float shown in FIG. 3.

FIG. 5 is a top view of the conductive strip of FIG. 3.

FIG. 9 is a graph depicting the output in ohms of a fluid level sensoraccording to the present invention in relation to float position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Referring now to FIG. 1, a diagrammatic illustration of a three wirefluid level sensor 10 according to the present invention is shown. Thesensor 10 includes float 12, contacts 14, conductor 16, segmentedconductor 18 comprised of conductive segments 18a-18h and resistorsR_(1`-R) ₇. Conductive segment 18h is shorted to conductor 16 when thefloat 12 is positioned at the lowest point of travel in a container (notshown). The contacts 14 attached to float 12 are positioned so that therelative vertical position of the contacts 14 corresponds with the fluidlevel in the container (not shown).

Functionally, in a typical application a fixed voltage reference signalis supplied between points A and B. The voltage measured at point C isused to proportionally determine the position of float 12. Since voltageinstead of resistance is the measured quantity, any variance in theresistance of resistors R₁ -R₇ will not affect the accuracy of thesensor 10. The voltage appearing at point C as float 12 moves from thebottom of the container, to the top of the container, corresponds to thecurve 40 shown in FIG. 6. Although the curve 40 represents ohms, in athree wire sensor such as sensor 10, the ohmage resistance willtranslate directly into voltage deviations for the three wirepotentiometer.

Resistors R₁ -R₇ can be chosen or selected to have values such that thevoltage present on any of the individual segments 18a-18h will providean accurate voltage corresponding to the amount of liquid remaining in acontainer, i.e. if the container is not rectangular in cross section,the value of the resistors may be chosen so that the voltage produced atany particular fluid level will correspond substantially with the actualvolume of fluid remaining in the container. For example, if thecontainer is hourglass shaped, resistors R₁ and R₂ as well as resistorsR₆ and R₇ would have larger ohmic values, whereas resistors R₃ -R₅ wouldhave lower ohmic resistance so that the voltage present on point C willhave a larger stair-step change when the float moves from a positionshorting segment 18h to conductor 16 to a position shorting segment 18gto conductor 16.

Referring now to FIG. 2, fluid level sensor 20 according to the presentinvention is shown. The sensor 20 includes nearly all of the componentsas sensor 10 of FIG. 1 with the difference being that sensor 20 is a twowire sensor or rheostat version. Sensor 20 is used in the identicalfashion as sensor 10 for sensing liquid levels in a container. Typicallythe sensor 20 is connected into a circuit as a series resistance atpoints B and C. As the position of float 12 changes with respect to thefluid level in the container, contacts 14 provide an electrical shortbetween one of the various segments 18a-18h and conduct 16. Thus theresistance for the sensor 20 varies from 0 ohms all the way up to thesum of the resistances of resistors R₁ -R₇.

As with the embodiment of FIG. 1, the sensor 20 of FIG. 2 can be"tailored" to a particular application so that the resistance betweenpoints B and C will accurately correspond to the actual amount of fluidremaining in a container by varying the values of resistors R₁ 14 R₇ tocorrespond proportionately to container cross-sectional contours.

Referring now to FIG. 3, a cross-sectional front elevational view of asensor 30 according to the present invention is shown. This embodimentof the sensor 30 incorporates the sensor 10 of FIG. 1 into a cylindricalhousing 22 having mounting flange 24 and bottom flange 26 retaining thesensor fixedly within the anti-slosh cylindrical housing 22. Housing 22,flange 24 and flange 26 are made of polypropylene. Flange 24 and flange26 include holes (not shown) which allow liquids to enter and leave thespace defined by the inner walls of housing 22.

Resistors R₁ -R₉ are surface mount device resistors. Such resistors arewell known in the art and may be made of ceramic/metallic mixturescommonly referred to as cermet thick film resistors or other resistivematerials suitable for exposure to the liquids which will contact theresistors. A thick film resistive paste is deposited onto a ceramicsubstrate and heated to a temperature sufficient to reflow or melt thethick film paste. Laser trimming of the resistors R₁ -R₉ is contemplatedas a convenient process for achieving desired resistance values. Theresistors are situated film side down on insulator 17 and soldertechniques are employed to attach the resistors to the conductivesegments 18a-18z. Hot plates, infrared ovens or soldering iron solderconnection technology well known in the art are contemplated astechniques useful in attaching the resistors mechanically andelectrically to the appropriate segments 18a-18z.

An aluminum strip 16 is the support member for the components of thesensor shown in FIG. 1. As in readily recognizable, the sensor 10 can beconfigured to any length, thus additional conductive segments 18x, 18yand 18z are shown in FIG. 3 to illustrate that the conductive segment 18may include a number of segments interconnecting corresponding resistorsin excess of the number shown in FIG. 1. The cutaway view of float 12reveals the location of contacts 14 provide an electrical short betweenthe location 15 and the aluminum conductor 16. Location 15 correpsondselectrically to the circuit node between resistors R₄ and R₅, which areconnected in series with the remaining resistors. Were the sensor 10located in a container, the position of the float 12 would correspondwith the liquid level in the container at location 15 where contact 14makes electrical contact with the conductive segment 18e.

Referring now to FIG. 5, a top view of the aluminum strip 16 is shownincluding the insulator 17 disposed on one surface of the strip 16.Resistor R₁ and conductive segments 18a and 18h are visible in this viewof the strip 16. Conductive strips 18a-18z conductive plastic conductorsilk screen deposited onto insulator 17. Insulator 17 is a plastic film,such as KAPTON film, a polyimide polymer manufactured by E. I. dupont deNemours, and is laminated to the aluminum strip with petroleum resistantadhesives. KAPTON film is resistant to attack by most petroleum basedfuels and is thus ideally suited for this application. The conductiveplastic conductor used for segments 18a-18z is manunfactured usingsilver or copper metal particles suspended in a plastic polymer. Such aproduct is available from Minico/Asahi Chemical of America, a thick filmmaterials supplier, located at 50 North Harrison Avenue, Congers, N.Y.10920.

An alternative approach for producing the conductive segments on aninsulator, known as flexible circuit manufacturing processes, is alsocontemplated. Flexible circuit manufacturing processes include the stepsof: 1) evaporation or sputter deposition of copper onto an insulator; 2)silk screen printing of an acid resistive material on the coppersurface; and 3) acid etching of the exposed copper to produce conductiverunners or conductive segments on the insulator surface. Both thick filmtechnology and flexible circuit technology are complemplated as viablealternatives for producing the insulator with conductive segments shownin FIGS. 3 and 5.

Referring now to FIG. 4, a top elevational view of the float 12 of FIG.3 is shown. The contacts 14 disposed within the hole 12a provide anelectrical short between one of the segments 18a-z and the conductor 16based upon the float position. The contacts 14 are connectedelectrically by a conductor 14a shown as a broken line in FIG. 4. Theirregular shape of hole 12a accommodates the profile of resistors R₁-R₉. The float 12 is typically made from nitrile rubber or other closedcell foam material which is resistive to liquid fuel absorption ordecomposition by fuels such as diesel fuel, gasoline, methanol etc.

As is apparent from the above description of the embodiments of theinvention, there are no constraints on the total length and resistanceof the sensors 10, 20 or 30 in any float height versus outputvoltage/resistance. Any voltage relationship or resistance relationshipcan be accommodated or created. Typical analog gasoline gauges when usedwith conventional signal damping circuitry will accept the stair-stepoutput signal from the sensors 10, 20, and 30 without significantmodification or change from the original design for use with a standardlinear resistance output fluid level sensor. Referring now to FIG. 3,connection terminals 26, 28 and 32 are electrically connected toconductive segment 18h, conductive segment 18a, and conductor 16,respectively to enable convenient connection to an analog or digitalfuel gauge (not shown). Thus a two or three wire potentiometric fluidlevel sensor is provided. It will be evident that the two wire sensor 20of FIG. 2 can be located in the cylindrical housing shown in FIG. 3. Fora two wire sensor, connecting to terminals 28 and 30 will provide anappropriate rheostat response signal or resistance.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. A liquid level sensor for providing a variableresistance corresponding to fluid level in a container, said sensorcomprising:a conductive strip having a first and a second surface; aninsulator attached to and substantially covering said first surface ofsaid strip; first, second, and third conductive segments mounted end toend on said insulator in non-touching substantially linear arrangement;a first resistive device attached to said insulator and electricallyconnected between said first conductive segment and said secondconductive segment; a second resistive device attached to said insulatorand electrically connected between said second conductive segment andsaid third conductive segment; a float including a hole therethough forreceiving said strip, said float disposed about said strip and movablein response to the fluid level in the container; electrical contactmeans attached to said float for electrically connecting said secondsurface of said strip and one of said first, second, or third conductivesegments when said float is positioned about said strip wherein saidfirst and second resistive devices are discrete resistors connected inseries.
 2. The sensor of claim 1 wherein said insulator is a plasticfilm.
 3. The sensor of claim 2 wherein said first, second, and thirdconductive segments are conductive plastic resistor ink.
 4. The sensorof claim 3 wherein said plastic film is KAPTON film.
 5. The sensor ofclaim 4 wherein said resistive devices are surface mount deviceresistors.
 6. The sensor of claim 1 including first terminal meanselectrically connected to said conductive strip for enabling anelectrical connection to said conductive strip and second terminal meanselectrically connected to said first conductive segment for enabling anelectrical connection to said first conductive segment.
 7. The sensor ofclaim 6 including a third terminal means electrically connected to saidthird conductive segment for enabling an electrical connection to saidfirst conductive segment.
 8. The sensor of claim 7 wherein saidinsulator is a plastic film and wherein said resistive devices aresurface mount device resistors.
 9. The sensor of claim 1 wherein saidfirst, second, and third conductive segments, and said insulator arecomponents of a flexible circuit assembly.
 10. A liquid level sensorcomprising:a conductive strip; an insulator mounted on said conductivestrip; at least three resistors attached to said insulator;non-continuous connection means for electrically connecting saidresistors in series; contact means for providing a short circuit betweensaid non-continuous connection means and said conductive strip; and afloat having said contact means attached thereto, said float positioningsaid contact means relative to the liquid level.
 11. The sensor of claim10 wherein said insulator is a plastic film.
 12. The sensor of claim 11wherein said float includes a hole therethrough for receiving saidconductive strip.
 13. The sensor of claim 12 wherein said non-contiguousconnection means is a conductive plastic resistor ink attached to saidplastic film.
 14. The sensor of claim 13 wherein said plastic film is apolyimide polymer.
 15. The sensor of claim 14 including a firstconnection terminal connected to said conductive strip, a secondconnection terminal attached to a first end of said resistors connectedin series, and a second connection terminal attached to a second end ofsaid resistors connected in series.
 16. The sensor of claim 10 whereinsaid insulator and said non-continuous connection means are componentsof a flexible circuit assembly.
 17. A liquid level sensor for providinga variable resistance corresponding to fluid level in a container, saidsensor comprising:an elongated conductive strip having a first andsecond surface; an insulator attached to and substantially covering saidfirst surface of said strip; at least three resistive devicessubstantially linearly arranged and attached to said insulator, saidresistive devices each having two electrical contact points; a pluralityof conductors attached to each of said electrical contact points of saidresistive devices and electrically connecting said resistive devices sothat each conductor completes a circuit between adjacent ones of saidresistive devices to form a series resistive circuit wherein all of saidresistive devices are connected in series; a float including a holetherethrough for receiving said strip; and electrical contact meansattached to said float for electrically connecting said second surfaceof said strip and one of said conductors when said strip is disposed insaid hole of said float.
 18. The sensor of claim 17 including agenerally cylindrical support tube and wherein said conductive strip ismounted within said tube.
 19. The sensor of claim 18 wherein said floatis made of nitrile rubber, said resistive devices are surface mountdevice resistors, said plurality of conductors are conductive plasticresistor ink, and said insulator is a plastic film made of polyimidepolymer.
 20. The sensor of claim 17 wherein said elongated conductivestrip is disposed substantially vertically within the container, saidsensor further including a first end conductor and a second endconductor, said series resistive circuit connected between said firstand second end conductors, said electrical contact means connecting saidfirst end conductor to said conductive strip when said float ispositioned substantially near the top of the container, and saidelectrical contact means connecting said second end conductor to saidconductive strip when said float is positioned near the bottom of thecontainer.
 21. The sensor of claim 17 wherein said insulator and saidplurality of conductors are components of a flexible circuit assembly.22. A liquid level sensor for detecting the level of liquid in acontainer, said sensor comprising:at least three discrete resistorsdisposed substantially vertically with respect to one another andsubstantially linearly; a plurality of connection conductors spacingapart and connecting said resistors to form a series resistive circuit;a ground conductor disposed in substantially parallel relationship withsaid series resistive circuit; a float disposed in close proximity tosaid series resistive circuit and movable in response to the level ofliquid in the container; and electrical contact means attached to saidfloat for electrically connecting said ground conductor and one of saidconnection conductors.
 23. The sensor of claim 22 including an insulatorand wherein said resistors and said ground conductor are attached tosaid insulator.
 24. The sensor of claim 23 wherein said float includes ahole therethrough and wherein said float is disposed about saidinsulator.